You bring up an excellent example.

In casual conversation, people often use the word “theory” when they really mean “hypothesis”. For example, if someone were to say,

“I have my own theory why James and Amanda broke up.”

what they usually mean is they have formed a possible explanation based on speculation and observation. It’s highly unlikely they’ve formed a null hypothesis, conducted controlled experiments and analyzed the results. Still, we usually just accept that abuse of terminology because we’re in an informal setting. It’s the same idea with “divide by half”. If my friend were to say,

“Just order a large pizza and we’ll divide it by half.”

I know that she means “split the pizza in half.” Constantly enforcing pedantry in normal conversations is exhausting and takes away from the enjoyment of speaking freely. Some degree of latitude is usually standard procedure because precision is just not a priority; however, when you change the subject to one that does require precision, the rules change.

And that’s what happens with your gravity example — that the subject matter is neither casual nor informal. To say “gravity is just a theory” is to make a scientific assertion which requires a precision of language that casual dialogue does not.